Although conventionally, a carbon-based raw material is generally used as the negative electrode of a lithium-ion secondary battery, in recent years, it has been often reported that such a lithium-ion secondary battery has produced abnormal heat and ignition (so-called thermal runaway). It is thought that the thermal runaway is produced partly because of a short-circuit within the battery. This is because, when a short-circuit occurs within the battery, an excessive inrush current flows toward the negative electrode, and thus heat is produced in the negative electrode and other members.
The internal short-circuit occurs probably because of an external impact or the breakage of a separator caused by a columnar metal lithium crystal (dendrite) precipitated on the surface of the negative electrode. The reason why the metal lithium crystal is easily precipitated in the lithium-ion secondary battery using the carbon-based raw material as the negative electrode is that the potential of the negative electrode is so low as to be 0.08 volt (vs. Li).
On the other hand, it is known that, as the negative electrode of the lithium-ion secondary battery, a spinel-type lithium titanate (S-LTO) is used, in addition to the carbon-based raw material. In the lithium-ion secondary battery using this negative electrode, since the potential of the negative electrode is so high as to be 1.55 volts (vs. Li), a metal lithium crystal is unlikely to be precipitated on the surface of the negative electrode, and thus it is possible to reduce the risk of the occurrence of an internal short-circuit; however, its negative electrode theoretical capacity is disadvantageously so low as to be about 175 mAh/g (in the carbon-based raw material, 372 mAh/g).
It is proposed that, as the negative electrode of the lithium-ion secondary battery, a bronze-type titanium oxide compound is used (for example, see patent document 1). In the lithium-ion secondary battery using this negative electrode, as in the case where the S-LTO is used, since the potential of the negative electrode is so high as to be about 1.5 volts (vs. Li), a metal lithium crystal is unlikely to be precipitated on the surface of the negative electrode, and thus it is possible to reduce the risk of the occurrence of an internal short-circuit; moreover, as compared with the case where the S-LTO is used, its negative electrode theoretical capacity can be increased up to 335 mAh/g.